The present invention relates to an apparatus for applying a thin film of a liquid coating medium onto a preferably curved surface of an article, preferably a glass bulb of a picture screen, having a rotatably drivable turntable on which a receptacle for the article to be coated is held fixed against relative rotation, a process pot held nonrotatably and surrounding the receptacle and the turntable, and a spray ring surrounding the surface to be coated and retained on the open end of the process pot.
In a known apparatus of this type, the glass bulb of a picture screen, for instance, is introduced into the process pot from the underside and pressed against a mask that surrounds the surface to be coated; the mask is intended to prevent the coating medium from reaching undesired regions of the glass bulb. This way of delivering and removing the glass bulb of the screen is mechanically complicated and inconvenient.
In another known apparatus, the glass bulb of the screen is introduced into the process pot from the top, and an open, split mask is brought in its closed position around the boundary edge of the surface to be coated. Once again, because of the split mask that has to be moved, this is mechanically complicated.
Both of the known apparatuses described have the common feature that especially at the edges of the surface to be coated, streaks and other discontinuities in the coating develop, unless special provisions for improved uniformity of the coating are made. This is the purpose of air guide elements placed against the corners of the mask in the known apparatuses. Nevertheless, such provisions do not prevent the flow from separating at the circumferential edges and becoming turbulent, which in turn means that the article to be coated becomes contaminated in undesired regions with the coating medium.
The object of the present invention is to provide an apparatus for applying a thin film to a surface of an article of the type described at the outset which is easier to handle as the article to be coated is being introduced and removed and which offers greatly improved uniformity of the film in the critical corner and edge regions of the surface to be coated.
To attain this object, in an apparatus for applying a thin film to an article as defined at the outset, the spray ring is provided with a central inflow opening and between the top part of the process pot and the spray ring, a first annular gap extending to beneath a fastening edge of the article to be coated on the receptacle is provided. The cross section of the annular gap is substantially less than an annular flow passage between the spray ring and the article to be coated retained in the receptacle. The receptacle is provided with a section cone which points toward a bottom of the process pot, and between which and an outer wall of the process pot there is a further annular gap and inside which a pressur relief chamber is formed. The pressure relief chamber is jointly defined by the bottom of the process pot. A laminar flow is aimed at the spray ring, or the top of the process pot, and the bottom of the process pot is joined to a suction flow.
By means of the provisions of the present invention, an air flow is produced which because of its different air velocities in various regions on the one hand makes the film to be applied uniform, even in the corner and edge regions of the preferably curved surface and on the other prevents contamination of other regions of the article to be coated. Because of the laminar flow in the region of the corners and edges of the preferably curved surface to be coated and because of the suction exerted on them toward the suction cone and the resultant downward-oriented flow component, on the one hand separation of the air flow at the edges and corners of the surface and thus turbulence are prevented, and on the other back spraying onto the surface to be coated is averted. In the further course of the air flow, more-effective suction of the atomized coating medium out of the critical region above the suction cone is achieved, after which the atomized coating medium can collect in the pressure relief chamber. In this region, the further gap prevents reverse turbulence in regions of the rotary bearings. On the other hand, the aspirated coating medium can be collected on the bottom of the process pot and removed by suction.
A vapor barrier from the pressure relief chamber to the bearing regions for the turntable is attained, without having to use vulnerable rotary seals and the pressure relief chamber is defined radially on the inside by an annular baffle between which and an inner wall of the process pot there is a third annular gap. It is expedient in this respect to secure the annular baffle to the underside of the turntable.
A process pot of low height is attained, which in its lower region surrounds the neck of, for instance, the glass bulb of the picture screen when the process pot is embodied cylindrically in its upper region and circularly-annularly in its lower region.
The outflow or removal by suction of the coating medium mist and/or coating medium droplets is advantageously achieved and the annular bottom of the process pot is curved in the circumferential direction and/or inclined in the radial direction and is joined to the suction flow via an opening in the region or regions of the lowest point or points.
To enable automatic or robot-controlled access to the article to be coated during its insertion and removal, the spray ring is retained in lowerable fashion on the process pot. Expedient features in this regard are obtained by the process pot having a removable top part, on which the spray ring is equipped retainers, which are distributed uniformly over the circumference and rest on one flange of the top part of the process pot, and is retained removably and which can be lowered into the bottom part of the process pot or can be lowered to below the upper end of the receptacle for the article to be coated. The result achieved is that the spray ring can be lowered down to or to below the height of the support of the article on the receptacle, so that the article to be coated can be grasped easily and moved through the spray ring.
In order, with the apparatus, to coat articles on their surface with the coating medium, such articles for instance being glass bulbs of pictures screens of various sizes or diagonal measurements, the receptacle for the article to be coated is retained interchangeably. Thus in an especially advantageous way, a receptacle suitable for a particular size of article to be coated can be retrieved from the process pot and replaced by a receptacle that can hold an article to be coated that is of different dimensions. Preferred features in this respect are obtained from the receptable being secured to the turntable and the turntable being interchangeably retained on a driven shaft which is hollow and is supported to rotate on a hollow stationary stand, which receives one end of the article to be coated and quick-action closures distributed uniformly over the circumference are provided between the turntable and one flange of the hollow shaft.
Thus in an especially advantageous way, a complete unit comprising the turntable, receptacle, suction cone and sealing cuff can be removed from the process pot and replaced by another one. To that end, the spray ring can be removed in a simple way from the top part of the process pot.
In a further feature of the present invention, the receptacle, on its support, is provided with a sealing cuff, which protrudes radially inward and on which a shoulder, remote from the surface to be coated, of the article to be coated rests sealingly. are provided, with which the surface to be coated of the article to be coated can be sealed off in a simple way from the other regions of the article to be coated. If the sealing cuff, in its radially inner region, is prestressed resiliently toward the article to be received a resiliently prestressed active contact of the sealing cuff with the shoulder of the article to be coated is achieved. The resilient prestressing can be achieved by means of concave curvature of the sealing cuff or by a kink provided along a circumferential line. To stabilize the sealing cuff, the sealing element rests on a resiliently prestressed lower part and is covered in the region of the fastening face by a retaining part so that the resilient prestressing is transmitted from a lower part onto the actual sealing element. In this respect, the sealing element protrudes radially inward past the resiliently prestressed lower part.
In a further feature of the present invention, provides the sealing cuff is fastened on the receptable by platelike components distributed over the circumference, and the platelike components are each provided with one protruding bolt, which bolts serve the purpose of detachable connection, in a manner fixed against relative rotation, to tabs disposed on a tension belt secured to the article on be coated which provides a fast and simple connection, in a manner fixed against relative rotation, attained between the article to be coated and the receptacle. Glass bulbs of picture screens, for instance, are provided on their circumference with a tension belt and retaining tabs secured to it, which are used for the aforementioned connection, in a manner fixed against relative rotation, with the bolts of the fastening plates.
Since the article to be coated on its surface is nonround, an example being a glass bulb of a picture screen which in plan view is rectangular with rounded corners, the corners located outside the inner circle meet the still air, so that in these corner regions severe turbulence and a shear flow arise, which carry the coating, already distributed outward by the spin coating process, still further away. To prevent this, and to improve the apparatus for the sake of uniformity of the film in the critical corner and edge region of the surface to be coated, in a variant the article to be coated on its surface, the surface being nonround in plan view, is at least partly surrounded by a guard baffle device is disposed sudstantially upright and extending to at least the plan of the surface to be coated.
Since essentially only the corner regions are threatened in this respect, is formed by individual guide baffles in the region of the corners of the articles the guide baffle device. It is expedient in this respect that the guide baffle, in terms of the direction of rotation, is disposed upstream in and downstream of the region of the corner, and the guide baffle portion upstream of the region of the corner is substantially longer than the guide baffle portion downstream of the region of the corner, because the side that is leading in the rotational direction relative to the corner region moves against the still air, while the trailing side is in the lee, as it were.
Thus with the aid of the guide baffle device, or the individual guide baffles, air turbulence can be kept away from the surface to be coated, for instance of the picture screen, to enable unhindered coating in the region of the corner.
To introduce and remove the article to be coated on its surface, retaining tabs are provided at the corners on the tension belt, as is realized for a glass bulb of a picture screen, for instance. To achieve the introduction and removal with the aid of a tool, such as a robot, it is possible that the guide baffle can be moved, for instant pivoted, out of the region of the corner of the article to bring the individual guide baffles out of this path of motion of the tool to the retaining tabs.
A simpler way of accomplishing access to the tool or robot to the individual retaining tabs in the corner region is obtained if the guide baffle device or guide baffle in the region of the corners or corner of the article has an outward-pointing recess surrounding a retaining tab of the article. With this baffle-like recess, constant access to the retaining tabs is assured, without requiring that the guide baffles be movable and without this recess having a negative effect on the guide baffle function.
A structurally advantageous embodiment is obtained by the guide baffle device or the individual guide baffles protruding past the surface to be coated.
Further details of the present invention can be learned from the ensuing description, in which the present invention is described and explained in further detail in terms of the exemplary embodiment shown in the drawing.